Optical subassemblies on which a laser chip and photodiode are mounted on a platform typically have the light admitting surface of the photodiode aligned with and directly facing the back facet of the laser chip to receive a portion of the light emitted through the back facet of the laser chip. Since only a small portion of the light emitted from the laser chip is emitted through the rear facet, it is important to capture a substantial portion of the light emitted from the laser chip rear facet as input to the photodiode. To maximize the amount of light emitted from the rear facet of the laser that is captured by the photodiode, the light admitting surface of the photodiode has been positioned in the cone of diverging emitted light from the rear facet of the laser chip. Positioning one of the larger light admitting surfaces of the photodiode parallel to the light emitting rear facet of the laser chip results in the largest amount of light falling on the active region of the photodiode.
One problem in positioning the photodiode in this manner is that the photodiode must be first mounted on a carrier, which is in turn mounted on the subassembly. The carrier mounted photodiode is typically substantially taller than the other components, and tends to be the constraint for residual package height. This increases the cost and complexity of the subassembly, and necessitates wire bonding around comers of the photodiode.